All three are correct and natural enough. In this context you can use either of them. But in other situations their uses will be slightly different;

- 降参 can also be used in more general situations. For example, when you're given a quiz and you can't answer it.
- When a nation surrenders, we always use 降伏. Perhaps 降伏 is defined as a diplomatic term, while 降参 is a general word and 投降 is more like a military term? (Just my two cents)

When a small group or an individual surrenders, there is no strict rule about the uses of these three.

By the way, I think people don't bother to use honorific forms when they're in war. I'd say "撃たないでくれ、[降参/降伏/投降]する!" (I'd also omit the subject). These points will contribute to the naturalness much more

降伏 is used when a group decides to surrender, admitting defeat, usually decided by a leader of the group.
投降 is when an individual (or soldiers without their leader) gives up and surrenders, against (or without asking) the will of the group that one is fighting with. (投 means "throw (away)" or "give up")
降伏 is used as an order or a word from the top, and you would suggest 「投降しなさい」 or give 「投降勧告」 to the losing side. TV dramas often show Japanese police saying 「ムダな抵抗はやめて速（すみ）やかに投降しなさい!」 to criminals.

I heard that many Japanese soldiers refused to 投降する, because they believed it would end up being killed by either the enemy soon or their own group's order later anyways for the betrayal. (don't know the truth.)

and, as Arui-san said, I'd probably just shout 「待って！降参！」 or just put my both hands up to show I mean no harm, but would never bother being polite